Automobile bumpers have evolved as relatively strong beams mounted on the front (and occasionally at the rear) of vehicles to transfer the forces of impact to the frame structure with a minimum of distortion of the sheet metal components of the body. Occasionally, they are mounted on shock-absorbing struts to dissipate the energy of impact. Except for the earliest vintage automobiles, the beam strength of the bumper results from the generally "C"-shaped frontal section having a relatively high moment of inertia in bending. This bending resistance is largely dependent upon the maintenance of the configuration of the cross-section, as any serious distortion of it is likely to vastly reduce the bending resistance by alteration of the sectional characteristics. For a long period, the bumper was essentially a piece of a steel formed into the desired configuration in a die with a large press. Comparatively recently, there has been a trend toward the manufacture of bumpers from molded thermoplastic materials. The structural shell has frequently been partially or fully closed at the rear to provide a stress-transfer web preventing the frontal "C"-shaped configuration from opening up like a clam shell on impact. These rear webs have been produced either through an extrusion process in which the full periphery of the cross-section extends throughout the length of the bumper, or by molding the bumper in two or more parts, followed by bonding these together through welding or adhesive processes. This type of construction is inherently more expensive, and there is a tendency for the parts to separate along the bonded seams under various conditions.